Add Insulation Over Existing Blown-In Insulation in an Attic Floor

Learn how to add insulation over existing blown-in insulation in an attic floor. Find out why you should use caution when adding insulation under an attic floor with wiring.

LESLIE: Wilson in Virginia has a question about insulation[1]. How can we help you today?

WILSON: Yes, I have a walkup attic and as you walk up to it – it was built in 1973 and as you walk up to the top of it there’s a flooring[2] been put down. It’s about 14 feet wide; seven foot on either side of center. And after you get off the flooring there’s just regular blown-in insulation[3]. I’m wondering if I can take that blown-in insulation and push it up under the floor and add new insulation[4]. After listening to your show for a few times I realize that I’m way under-insulated[5] and I need to put some r27 or better up there and I’m just wondering if I can push that stuff up underneath the flooring – not tight but just loose – and put new stuff down or whether I should rip it all out altogether or just lay stuff on top of it altogether.

TOM: In other words, you’re not going to be using this space above the attic floor except for unheated storage.

WILSON: Right. There will be nothing upstairs that I have to worry about.

TOM: OK. Well, with that being said, how do you know that there’s no insulation under the floor section that’s there right now?

WILSON: Well there is but it’s that blown-in insulation.

TOM: OK.

WILSON: And it has settled down to about three inches on the outside of the floor so I figured it must be settled down two or three inches on the inside; you know, up underneath it. So I didn’t know whether I’d be safe to push up what was there up underneath it just to give it …

TOM: I think that you could do some of this. If you could actually reach back in there you probably could very carefully but remember, you’ve got wires in there and other things like that. You could cause some damage. I would be more concerned in getting insulation in the areas that were more accessible; like kind of – my theory is here let’s get as much insulation as we possibly can in the areas that it’s practical to do that.

LESLIE: So not try to take like a fiberglass bat and slide it underneath?

TOM: Definitely not a fiberglass bat. He may be able to get the blown-in. Or you could drill some holes in that floor and get some new blown-in and actually blow it in yourself. You can rent those blowers …

WILSON: Right.

TOM: … usually from the home centers or from the rental houses.

WILSON: OK.

TOM: Now as far as the rest of the area is concerned, generally what you want to do is fill up the space between the floor joist to the top of the joist and if you have enough blown-in to do that, fine; and then add a second set of fiberglass bats perpendicular to the floor joist. So if your floor joists are like eight inches and you add like another 10-inch fiberglass bat on top of that, you know, now you’re going to have maybe 18 inches or so. So times three, you’ve got yourself like almost 60r in insulation, which is pretty good.

WILSON: OK, because I was planning on getting some of those pink fiberglass rolls and just rolling it out …

LESLIE: Mm-hmm.

WILSON: … and rolling it that way.

TOM: Right.

WILSON: But I didn’t know what to do with the loose stuff.

TOM: (overlapping voices) Either rolls or the bats. Either the rolls or the bats. Either are fine. Unfaced.

WILSON: Unfaced.

TOM: Unfaced.

LESLIE: Make sure you wear long sleeves and a dust mask and safety glasses.

WILSON: If I want to do – if I want to cover up two-thirds of the floor area also, should I use unfaced?

TOM: Yes.

WILSON: Because there are two-thirds of the area that I’m not even using.

TOM: Yes, unfaced.

WILSON: Unfaced all the way?

TOM: Yep, unfaced. Because if you put a face insulation in there you’re going to trap moisture on the wrong side one way or the other.